How to quantify the reflectance in an image?
You should first know about the layer-wise working of a convolutional neural network. Read this https://distill.pub/2018/building-blocks/

Each layer of a cnn forms representations that are increasingly complex , which are  combinations of simpler representations formed by its previous layers. 
"shine" is not a pattern (from image processing perspective). shine occurs due to change in intensity values. 

And for quantifying "shine" you dont have to go for convolutional networks only , there are many more simpler methods for such tasks you could try fitting simple linear model and progress towards more complex ones.

and for classifying textures , there is interesting work going with "invariant scattering convolutional networks" you should check that.